The Great Roman Civil War (49–45 BC), also known as Caesar's Civil War, was one of the last politico-military conflicts in the Roman Republic before the establishment of the Roman Empire. It began as a series of political and military confrontations, between Julius Caesar (100–44 BC), his political supporters (broadly known as Populares), and his legions, against the Optimates (or Boni), the politically conservative and socially traditionalist faction of the Roman Senate, who were supported by Pompey (106–48 BC) and his legions.[1]

After a five-year-long (49–45 BC) politico-military struggle, fought in Italy, Illyria, Greece, Egypt, Africa, and Hispania, Caesar defeated the last of the Optimates in the Battle of Munda and became Dictator perpetuo (Dictator in perpetuity) of Rome.[2] The changes to Roman government concomitant to the war mostly eliminated the political traditions of the Roman Republic (509–27 BC) and led to the Roman Empire (27 BC–AD 476).

The First Triumvirate (so denominated by Cicero), comprising Julius Caesar, Crassus, and Pompey, ascended to power with Caesar's election as consul, in 59 BC. The First Triumvirate was unofficial, a political alliance the substance of which was Pompey's military might, Caesar's political influence, and Crassus' money. The alliance was further consolidated by Pompey's marriage to Julia, daughter of Caesar, in 59 BC. At the conclusion of Caesar's first consulship, the Senate (rather than granting him a provincial governorship) tasked him with watching over the Roman forests. This job, specially created by his Senate enemies, was meant to occupy him without giving him command of armies, or garnering him wealth and fame.

Caesar, with the help of Pompey and Crassus, evaded the Senate's decrees by legislation passed through the popular assemblies. By these acts, Caesar was promoted to Roman Governor of Illyricum and Cisalpine Gaul. Transalpine Gaul (southern France) was added later. The various governorships gave Caesar command of an army of (initially) four legions. The term of his proconsulship, and thus his immunity from prosecution, was set at five years, rather than the customary one year. His term was later extended by another five years. During this ten-year period, Caesar used his military forces to conquer Gaul and invade Britain, without explicit authorisation by the Senate.

Roman world in 56 BC, when Caesar, Crassus and Pompey meet at Luca for a conference in which they decided: to add another five years to the proconsulship of Caesar in Gaul; to give the province of Syria to Crassus and both Spains and Africa to Pompey

In 52 BC, at the First Triumvirate's end, the Roman Senate supported Pompey as sole consul; meanwhile, Caesar had become a military hero and champion of the people. Knowing he hoped to become consul when his governorship expired, the Senate, politically fearful of him, ordered he resign command of his army. In December of 50 BC, Caesar wrote to the Senate agreeing to resign his military command if Pompey followed suit. Offended, the Senate demanded he immediately disband his army, or be declared an enemy of the people: an illegal political bill, for he was entitled to keep his army until his term expired.

A secondary reason for Caesar's immediate want for another consulship was delaying the inevitable senatorial prosecutions awaiting him upon retirement as governor of Illyricum and Gaul. These potential prosecutions were based upon alleged irregularities that occurred in his consulship and war crimes committed in his Gallic campaigns. Moreover, Caesar loyalists, the tribunesMark Antony and Quintus Cassius Longinus, vetoed the bill, and were quickly expelled from the Senate. They then joined Caesar, who had assembled his army, whom he asked for military support against the Senate; agreeing, his army called for action.

In 50 BC, at his Proconsular term's expiry, the Pompey-led Senate ordered Caesar's return to Rome and the disbanding of his army, and forbade his standing for election in absentia for a second consulship; because of that, Caesar thought he would be prosecuted and rendered politically marginal if he entered Rome without consular immunity or his army; to wit, Pompey accused him of insubordination and treason.

On 10 January 49 BC, leading one legion, the Legio XIII Gemina, General Julius Caesar crossed the Rubicon River, the boundary between the Cisalpine Gaul province to the north and Italy proper to the south, a legally proscribed action forbidden to any army-leading general. The proscription protected the Roman Republic from a coup d'état; thus, Caesar's military action began a civil war.

This act of war on the Roman Republic by Caesar led to widespread approval amongst the Roman civilians, who regarded him as a hero. The historical records differ about which decisive comment Caesar made on crossing the Rubicon: one report is Alea iacta est (usually translated as "The die is cast").

Column of Julius Caesar, where he addressed his army to march on Rome and start the Civil War, Rimini, Italy

Caesar's March on Rome was a triumphal progress; yet, the Senate, ignorant of Caesar's being armed only with a single legion, feared the worst and supported Pompey, who, on grasping the Republic's endangerment, said: "Rome cannot be defended", and escaped to Capua with his politicians, the aristocratic Optimates and the regnant consuls; Cicero later characterised Pompey's "outward sign of weakness" as allowing Caesar's politico-military consolidation to achieve Roman dictatorship.

Despite having retreated, in central Italy, Pompey and the Senatorial forces disposed of at least two legions, some 11,500 soldiers (he earlier had ordered Caesar return to Italy from Gaul), and some hastily levied Italian troops commanded by Lucius Domitius Ahenobarbus (Domitius). As Caesar progressed southwards, Pompey retreated towards Brundisium, initially ordering Domitius (engaged in raising troops in Etruria) to stop Caesar's movement on Rome from the direction of the Adriatic seaboard.

Belatedly, Pompey ordered Domitius to retreat south also, and make junction with Pompey's forces. Domitius mostly ignored Pompey's orders, and, after being isolated and trapped near Corfinium was forced to surrender almost thirty cohorts of troops (about three Legions), most of whom promptly joined Caesar's army.

Pompey escaped to Brundisium, there awaiting sea transport for his legions, to Epirus, in the Republic's eastern Greek provinces, expecting his influence to yield money and armies for a maritime blockade of Italy proper. Meanwhile, the aristocrats (the Optimates)—including Metellus Scipio and Cato the Younger—joined Pompey there, whilst leaving a rear guard at Capua.

Caesar pursued Pompey to Brundisium, expecting restoration of their alliance of ten years prior; to wit, throughout the Great Roman Civil War's early stages, Caesar frequently proposed to Pompey that they, both generals, sheathe their swords. Pompey refused, legalistically arguing that Caesar was his subordinate and thus was obligated to cease campaigning and dismiss his armies before any negotiation. As the Senate's chosen commander, and with the backing of at least one of the current consuls, Pompey commanded legitimacy, whereas Caesar's military crossing of the Rubicon River frontier rendered him a de jure enemy of the Senate and People of Rome. Nevertheless, in March 49 BC, Pompey escaped Caesar at Brundisium, fleeing by sea to Epirus, in Roman Greece.

Returned to Rome in December of 49 BC, Caesar was appointed Dictator, with Mark Antony as his Master of the Horse. Caesar kept his dictatorship for eleven days, tenure sufficient to win him a second term as consul with Publius Servilius Vatia Isauricus as his colleague. Afterwards, Caesar renewed pursuit of Pompey, then in Roman Greece.

From Brundisium, Caesar with 7 legions crossed the Strait of Otranto to the gulf of Valona, not Palaesta in Epirus (modern Palase/Dhermi, Albania), as reported by Lucan.[3] In that time, Pompey considered three courses of action: (i) alliance with the King of Parthia, an erstwhile ally, far to the east; (ii) invade Italy with his naval superiority; and (iii) confronting Julius Caesar in decisive battle. A Parthian alliance was unfeasible, a Roman general fighting Roman legions with foreign troops was craven; and the military risk of an Italian invasion was politically unsavoury, because, the Italians (who thirty years earlier had rebelled against Rome) might rise against him; thus, on councilor's advice, Pompey decided to fight Julius Caesar in decisive battle.

Moreover, Caesar's pursuing him to Illyrium, across the Adriatic Sea, decided the matter, and, on 10 July 48 BC, Pompey fought him in the Battle of Dyrrhachium, costing Caesar 1,000 veteran legionaries and a retreat. Disbelieving that his army had bested Caesar's legions, Pompey misinterpreted the retreat as a feint to a trap, and refused to give chase for the decisive, definitive coup de grâce, thus losing the initiative, and the chance to quickly conclude Caesar's Civil War; meanwhile, Caesar retreated southwards. Near Pharsalus, Caesar pitched a strategic bivouac, and Pompey attacked, yet, despite his much larger army, was conclusively defeated by Caesar's troops. A major reason for Pompey's defeat was a miscommunication among front cavalry horsemen.

Pompey fled to Egypt, where he was murdered by an officer of King Ptolemy XIII. Caesar pursued the Pompeian army to Alexandria, where they camped and became involved with the Alexandrine civil war between Ptolemy and his sister, wife, and co-regnant queen, the PharaohCleopatra VII. Perhaps as a result of Ptolemy's role in Pompey's murder, Caesar sided with Cleopatra; he is reported to have wept at the sight of Pompey's head, which was offered to him by Ptolemy's chamberlain Pothinus as a gift.

After spending the first months of 47 BC in Egypt, he went to Syria, and then to Pontus to deal with Pharnaces II, a client king of Pompey's who had taken advantage of the Romans being distracted by their civil war to oppose the Roman-friendly Deiotarus and make himself the ruler of Colchis and lesser Armenia. At Nicopolis he had defeated what little Roman opposition Caesar's lieutenant, the governor of Asia Gnaeus Domitius Calvinus, could muster. He had also taken the city of Amisus, which was a Roman ally, made all the boys eunuchs, and sold the inhabitants to slave traders. After this show of strength against the Romans, Pharnaces drew back to suppress revolt in his new conquests.

Nevertheless, the extremely rapid approach of Caesar in person forced Pharnaces to turn his attention back to the Romans. At first, recognizing the threat, he made offers of submission, with the sole object of gaining time until Caesar's attention fell elsewhere; Caesar's speed brought war quickly and battle took place near Zela (modern Zile in Turkey), where Pharnaces was routed with just a small detachment of cavalry. Caesar's victory was so swift and complete that, in a letter to a friend in Rome, he famously said of the short war, "Veni, vidi, vici" ("I came, I saw, I conquered"): indeed, for his Pontic triumph, that may well have been the label displayed above the spoils.

Pharnaces himself fled quickly back to the Bosporus, where he managed to assemble a small force of Scythian and Sarmatian troops, with which he was able to gain control of a few cities; however, a former governor of his, Asandar, attacked his forces and killed him. The historian Appian states that Pharnaces died in battle; Dio Cassius says Pharnaces was captured and then killed.

Caesar returned to Rome to deal with several mutinous legions. While Caesar had been in Egypt installing Cleopatra as Queen, four of his veteran legions encamped outside of Rome under the command of Mark Antony. The legions were waiting for their discharges and the bonus pay Caesar had promised them before the battle of Pharsalus. As Caesar lingered in Egypt, the situation quickly deteriorated. Antony lost control of the troops and they began looting estates south of the capital. Several delegations of diplomats were dispatched to try to quell the mutiny.

Nothing worked and the mutineers continued to call for their discharges and back pay. After several months, Caesar finally arrived to address the legions in person. Caesar knew he needed these legions to deal with Pompey's supporters in north Africa, who had mustered 14 legions of their own. Caesar also knew that he did not have the funds to give the soldiers their back pay, much less the money needed to induce them to reenlist for the north African campaign.

When Caesar approached the speaker's dais, a hush fell over the mutinous soldiers. Most were embarrassed by their role in the mutiny in Caesar's presence. Caesar asked the troops what they wanted with his cold voice. Ashamed to demand money, the men began to call out for their discharge. Caesar bluntly addressed them as "citizens" instead of "soldiers," a tacit indication that they had already discharged themselves by virtue of their disloyalty.

He went on to tell them that they would all be discharged immediately. He said he would pay them the money he owed them after he won the north African campaign with other legions. The soldiers were shocked. They had been through 15 years of war with Caesar and they had become fiercely loyal to him in the process. It had never occurred to them that Caesar did not need them.

The soldiers' resistance collapsed. They crowded the dais and begged to be taken to north Africa. Caesar feigned indignation and then allowed himself to be won over. When he announced that he would suffer to bring them along, a huge cheer arose from the assembled troops. Through this reverse psychology, Caesar reenlisted four enthusiastic veteran legions to invade north Africa without spending a single sesterce.

Caesar quickly gained a significant victory at Thapsus in 46 BC over the forces of Metellus Scipio, Cato the Younger and Juba (who all committed suicide).

January 1: The Roman Senate receives a proposal from Julius Caesar that he and Pompey should lay down their commands simultaneously. The Senate responds that Caesar must immediately surrender his command.

January 10: Julius Caesar leads his army across the Rubicon, which separates his jurisdiction (Cisalpine Gaul) from that of the Senate (Italy), and thus initiates a civil war.

February, Pompey's flight to Epirus (in Western Greece) with most of the Senate

November: Caesar leaves for Farther Hispania to deal with a fresh outbreak of resistance.

Caesar, in his role as Pontifex Maximus, reforms the Roman calendar to create the Julian calendar. The transitional year is extended to 445 days to synchronize the new calendar and the seasonal cycle. The Julian Calendar would remain the standard in the western world for over 1600 years, until superseded by the Gregorian Calendar in 1582.

Caesar was later proclaimed dictator first for ten years and then in perpetuity. The latter arrangement in openly doing away with a term limit, triggered the conspiracy leading to his assassination on the Ides of March in 44 BC out of such fears. Following this, Antony and Caesar's adopted son Octavius (later Caesar Augustus) had to fight yet another civil war against remnants of the Optimates and Liberatores faction, who were crushed by the skill of Marcus Antonius, who was able to defeat his two main opponents, and Octavius, despite having his camp overrun, evaded capture.